In the case of two recording systems, such computed tomography scanners are also known as dual-source CT systems. Here, a recording system is understood to mean the combination of an X-ray source and an opposing X-ray detector. Dual-source CT systems are used, inter alia, for cardiac recordings. In order to optimize the time resolution in EKG-synchronized cardiac recordings, data from a quarter-rotation (in parallel geometry) of the rotating frame is used per recording system for generating a CT image. Compared to CT systems with only one recording system, the time resolution is doubled as a result of this. In the vicinity of the center of the rotation, the time resolution using a dual-source CT system is a quarter of the rotational time of the rotating frame. However, the image reconstruction from the measurement data of the two recording systems assumes that both systems are operated at the same X-ray energy spectrum in order to obtain an artifact-free image.
If the two recording systems are operated at different X-ray energies or X-ray energy spectra, as is the case in EKG-synchronized dual-energy recordings, two quarter-rotation data records cannot simply be combined to generate an image since the data is inconsistent as a result of the different X-ray energy spectra, and the different mean X-ray energies resulting from this. The result of this inconsistency is artifacts and CT value displacements in CT images where the semi-rotation for acquiring the measurement data, as a minimum requirement for image reconstruction, is composed of a quarter-rotation per recording system. Therefore, EKG-synchronized dual-energy recordings using a dual-source CT system previously utilized measurement data of a semi-rotation per recording system which suffice for generating a separate image per recording system. The two separate images were then suitably combined to form a mixed image. However, as a result of this, the time resolution is only half of the rotational time of the rotating frame and so the actual advantage of using dual-source CT systems, namely the generation of EKG-synchronized images with optimized time resolution of a quarter of the rotational time, is lost.
The subsequently published document DE 10 2008 051 043, the entire contents of which are hereby incorporated herein by reference, describes a method for optimizing the time resolution in EKG-synchronized dual-energy dual-source CT recordings. Herein, the inconsistent measurement data of the two recording systems recorded at different X-ray spectra are decomposed into a spatial low-frequency component and a spatial high-frequency component. The frequency boundary is selected such that relevant movements of the coronary arteries are substantially contained in the spatial high-frequency component of the data and, by contrast, the inconsistencies produced by the different mean energy of the measurement data are substantially contained in the spatial low-frequency component. The low-frequency data generates an image with at least one semi-rotation per recording system, whereas only a quarter-rotation per recording system is used for the diagnostically relevant high-frequency measurement data in order to optimize the time resolution. However, the time resolution of the image in this method is dependent on the spatial frequency of the imaged structures and a clean separation between spatial high-frequency moved structures, especially coronary arteries, and low-frequency artifacts is not always ensured.